Rails 3: migration error when using json as a column type in an ActiveRecord backed by Postgres

Change your migration like

class CreateThing < ActiveRecord::Migration
  def change
    create_table :things do |t|
      t.integer :user_id
      t.column :json_data, :json   # Edited
      t.timestamps
    end
    add_index :things, :user_id
  end
end

And by default rake db tasks will look into schema.rb( which wont be the case for postgres) so in application.rb change it to

config.active_record.schema_format = :sql